You can load MP3s from any non-copy-protected source. This includes sites that offer free or non-DRM tunes like Rhapsody and Amazon or your fave band site. But you do need to load them into iTunes if you want to sync them to the iPhone (since that's the app used to sync the phone). Generally it's pretty painless, double-click on the tune if iTunes is the default media player and it will automatically add them to iTunes. Oherwise, right-click and select open with iTunes. Once there you can select them for syncing to the iphone.

Cut and paste will likely have to come from Apple and not a 3rd party. At this point, I hope Apple has a clue that folks want this feature! With the future release of Office suites, it really will be a must.